Taking Phentermine With Robitussin DM

In our latest question and answer, the pharmacist discusses safety concerns when combining phentermine with Robitussin DM.

Jan 08, 2018

jane asked

Is it safe to take phentermine 37.5 with Robitussin max strength cough and chest congestion DM?


There are no listed or reported interactions between phentermine and Robitussin Max Cough and Chest Congestion (dextromethorphan and guaifenesin). They are considered to be safe when taken in combination.

Potential Phentermine Robitussin DM Interaction

Theoretically, the combination of phentermine and dextromethorphan could potentially increase levels of the neurotransmitter serotonin in the brain due to each drugs respective mechanism of action. However, there have been no reports of adverse effects. 

Phentermine is known to increase the release of norepinephrine and dopamine primarily. High doses of phentermine has shown to increase the release of serotonin, but the effect appears to be minimal according to studies.

At high doses, dextromethorphan, the cough suppressant in Robitussiin DM can also increase serotonin levels but again, this appears to be minimal. Taking both dextromethorphan and phentermine at indicted dosage levels should not cause problems. However, if you are on additional medications that increase serotonin, such as certain antidepressants, you may want to avoid combining them due to the risk of serotonin syndrome.

Serotonin syndrome is characterized by a variety of symptoms including:

  • Hyperthermia
  • Rigitity
  • Mood Changes
  • Delerium

Serotonin syndrome is very rare, but the risk is increased when taking a combination of drugs that affect serotonin. Again, the combination of dextromethorphan and phentermine has not been reported as causing serotonin syndrome but caution is advised if taking other serotonergic medications.

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